1. Field of the Invention
The present invention concerns liquid crystal compounds and mixtures as well as electro-optical devices containing same. The invention also concerns method of manufacturing these liquid crystal compounds.
2. Background Description
Liquid crystals are useful primarily as dielectrics in indicating devices, since the optical properties of such substances can be influenced by an electric field. Electro-optical devices based on liquid crystals are well known to the person skilled in the art and can be based on various effects such as, for example, dynamic scattering, the deformation of aligned phase (DAP type), the Schadt-Helfrich effect (rotation cell), the guest/host effect ("guest/host cell") or a cholesteric-nematic phase transition.
Since, in general, it is not possible to achieve all desired properties, such as, for example, high chemical and photochemical stability, low viscosity, large nematic or cholesteric mesophase ranges, short response times and the like, with a single compound, mixtures of several components are usually used. As a rule, such mixtures mainly contain compounds having a low viscosity, but these compounds on the other hand usually also have low clearing points. For this reason, substances which increase the clearing point must be added to the mixtures.
Liquid crystals have also previously been used as stationary phases in gas chromatography. However, the hitherto known liquid crystalline stationary phases generally have the disadvantage that they can be used only up to temperatures of about 180.degree. C.
It has now been found that the compounds of the invention form very large mesophase ranges with high clearing points and that they generally exhibit a nematic mesophase in the entire liquid crystalline range or at least in a large part of it. They have a good chemical and photochemical stability and they are colourless. The viscosity values are comparatively low taking into consideration the high clearing points. Those compounds of formula I in which R.sup.2 signifies cyano or p-cyanophenyl have a positive dielectric anisotropy; the remaining compounds of formula I (i.e. those in which R.sup.2 signifies p-alkylphenyl or trans-4-alkylcyclohexyl) have small absolute values of the dielectric anisotropy.
On the basis of their very good separation capability and their high clearing points and boiling points the compounds of the invention are excellently suited for or as stationary phases in gas chromatography and can also be used at very high working temperatures. The compounds in accordance with the invention are especially suitable for the separation of isomer mixtures which can not be separated or which can be separated only with difficulty using conventional stationary phases, such as, for example, cis/trans-isomeric cyclopentanes and cyclohexanes (e.g. 1,4-disubstituted cyclohexanes), isomeric aromatic hydrocarbons (e.g. o-, m- and p-disubstituted benzenes, anthracene/phenanthrene), double bond isomers (e.g. unsaturated fatty acid esters) and the like. The compounds in accordance with the invention are also especially suitable for use in capillary columns. Further, the compounds of formula I in which X signifies the group --CH.sub.2 CH.sub.2 -- are also especially suitable for the separation of silylated samples. Such samples of silyl derivatives (e.g. trimethylsilyl derivatives) are frequently prepared when mixtures of compounds having acidic hydrogen atoms (e.g. acids, alcohols, phenols, amines, amides) are present in order to facilitate the separation.
Furthermore, the compounds of the invention are suitable as clearing point-increasing substances for liquid crystalline dielectrics. In this case they primarily have the advantage that either very large clearing point increases can be achieved or on the other hand only comparatively small amounts need be added to achieve a particular clearing point increase and thus the remaining properties of the mixture are altered only insignificantly.